Ferroelectrics offer the possibility of many important electronic devices, especially dense, non-volatile memories. The ferroelectric material can be electrically switched between two stable electrical polarization states. The resultant electrical field also has two states. It persists in the absence of the switching field and can be used for various purposes. The most widespread application is a non-volatile memory in which the ferroelectric fills the gap of a thin-film capacitor. Depending on the polarity of the writing pulse, the capacitor is charged to one of two voltage states of opposite polarity. Unlike semiconductor capacitive memories (dynamic random access memories or DRAMs), the ferroelectric capacitive memory is non-volatile and maintains its stored information even if depowered. Also, because of the very high effective dielectric constants of ferroelectrics, ferroelectric memories can be made very small.
Until recently, ferroelectric memories mostly relied on ferroelectric thin films between metallic electrodes. Because the ferroelectric was deposited on the polycrystalline metal, it was also polycrystalline. Its polycrystalline structure introduced substantial problems with reliability and aging because of the significant interfacial effects at the grain boundaries.
Recently, however, crystalline ferroelectric thin-film devices have been reported. In U.S. Pat. No. 5,168,420, I disclosed the growth of a crystalline ferroelectric thin film of lead zirconate titanate (PZT) on a crystalline layer of the cuprate perovskite high-temperature superconductor YBa.sub.2 Cu.sub.3 O.sub.7-x (YBCO), which acted as the lower electrode of the capacitor. Another YBCO layer formed the upper electrode. YBCO can be grown to have high crystalline quality with a c-axis orientation. Its a- and b-axis lattice parameters are 0.383 and 0.393 nm while its c-axis parameter is 1.168 nm, and all its axes are approximately perpendicular. Thus, a c-axis orientation produces a layered structure. I now believe that the high crystalline quality of the PZT was due to the layered perovskite on which it was grown. In U.S. Pat. No. 5,155,658, I suggested that the YBCO/PZT/YBCO structure could be grown on silicon substrates by use of an intermediate buffer layer of yttria-stabilized zirconia (YSZ). Thereby, the ferroelectric memory could be integrated with silicon support circuitry. However, YBCO is disadvantageous in that its crystalline growth requires temperatures of nearly 800.degree. C., which is incompatible with silicon processing and, when used as the upper electrode, severely limits the choice of ferroelectrics, which tend to dissociate at those temperatures. Furthermore, the layered structure of perovskite electrodes, typical for high-temperature superconductors complicates the design.
Others have suggested that cubic metal oxide electrodes be used for ferroelectric capacitors. One such oxide is La.sub.1-x Sr.sub.x CoO.sub.3, with 0.ltoreq.x&lt;1 (LSCO), which grows with almost singly crystalline quality at around 600.degree.-650.degree. C. Other examples are LaCrO.sub.3 and SrRuO.sub.3. However, these cubic metal oxides do not grow with satisfactory crystalline quality on YSZ-buffered silicon. In U.S. patent application Ser. No. 07/925,350, filed Aug. 4, 1992 and incorporated herein by reference, I disclosed that singly crystalline metal oxide can be grown on YSZ-buffered silicon by use of an intermediate template layer of a layered perovskite, such as bismuth titanate (Bi.sub.4 Ti.sub.3 O.sub.12 or BTO). The PZT or other ferroelectric then epitaxially grows on the cubic metal oxide. Layered perovskites appear to exhibit a powerful tendency to grow with a c-axis orientation, that is, with the long axis perpendicular to the film. The crystallinity is optimized when the template layer is grown to a thickness of 20-40 nm is a temperature range of 600.degree.-690.degree. C., optimally around 640.degree. C. This orientational preference appears to follow from the low surface energy of the nearly square a-b face of the layered perovskites. Furthermore, the a-b face has dimensions and crystal chemistry that are nearly identical to those of the cubic perovskite oxides. The layered perovskite can be YBCO or preferably bismuth titanate, which seems to act as an especially powerful template.
Nonetheless, depositing a buffer layer of YSZ on silicon prior to forming the ferroelectric elements is not totally satisfactory. The silicon substrate is advantageous in that silicon support circuitry, especially complementary metal-oxide-semiconductor (CMOS) circuitry, can be fabricated in it. The YSZ is deposited at a temperature around 800.degree. C., which is incompatible with silicon CMOS processing. It would be especially advantageous if the ferroelectric elements could be grown on silicon dioxide, which is an amorphous glass. The silicon oxide would isolate the ferroelectric elements and could be used as the oxide layer in a metal-oxide-semiconductor (MOS) gate transistor associated with each ferroelectric memory cell. Unfortunately, LSCO grown directly on SiO.sub.2 shows very little crystallographic orientation and appears to be polycrystalline. Also, PZT grown directly on the SiO.sub.2 forms in the non-ferroelectric pyrochlore phase.